How to reproduce:
1- Enable "Show Objective" via Map.
2- Get a quest showing a POI on map.
3- Open tracking on minimap with right click button. (do not close, do not select anything)
4- Open minimap with the M keybind.

If you can't reproduce, reload your UI and try again step 3 and 4.

I'm also having random lua error sometime DropDownMenu.lua at line 656, UIDROPDOWNMENU_MENU_LEVEL being nil, when I play with unitframes dropdown. :/

I would like to know if some addon authors have the same issues since a couple of beta builds on beta/ptr.

No addons loaded at all, just with this minimap setscript. How to 100% reproduce:

Make a pre-made, alliance side. Go to stowmwind castle, a MoP intro movie should play, skip it. There is a guy in front of you with a quest, take it. Now, right-click minimap to show dropdown, leave it open and hit "M".

Look like it was something in my WTF folder causing these issue. How can something in WTF folder can affect a dropdown ...

Maybe it was something in a saved variables file that got loaded. This is one of the reasons why "delete/rename Interface and WTF folder (and sometimes Cache)" is the first step to debugging.

__________________
"You'd be surprised how many people violate this simple principle every day of their lives and try to fit square pegs into round holes, ignoring the clear reality that Things Are As They Are." -Benjamin Hoff, The Tao of Pooh

I just got this same error in 5.0.4 on the live server. I discovered that if I renamed the WTF\Config.wtf file and let the game create a new one, the problem went away. Looking at it a bit closer, I found that I could just remove one line from my original Config.wtf file and not get the error any more. That one line was:
SET taintLog "1"

Using only the same test addon that Tukz used in the first post in this thread, I get the same Lua error on the live server after 5.0.4 was released. Sometimes the game will crash to the desktop with an exception error that I can submit to Blizzard.

My steps:
1. Enable only the test addon
2. Created new Alliance human priest.
3. Enter the world, and press ESC to skip the movie.
4. Type: /console taintLog 1
5. Type: /reload
6. Right click on minimap to show the menu.
7. While the menu is open, press M to open the map.
8. At this point it either crashes to desktop, or I get a Lua error.

When taintLog is set to 0, there is no error or crash when the same steps are performed.

I would avoid the built in drop down system to be honest, I only felt my innocent addons breaking important UI functionality because they tainted and broke the secure environment, so in the long run, and looking at how often blizzard use drop downs, I think it's smart to perhaps make your own drop down system or use some library for such, ugh. :/

Yeah, the Blizzard dropdown system is anything but bug free. The problem in this instance is that the bug generated is IN the Blizzard Lua code!

It appears as others have reported thought that the issue magically disappears when taintlogging is turned off with /console taintlog 0. But the problem with that is that I don't have a taintlog anymore... -_-

But the fact that it somehow is related to taintlogging, and that the errormessage I've been getting the most claims that 'UIDROPDOWNMENU_MENU_LEVEL' suddenly isn't a number anymore, but a userdata value (raw block of memory pretty much), suggests that the problem might be in Blizzard's C code, and not in the Lua code.

I would avoid the built in drop down system to be honest, I only felt my innocent addons breaking important UI functionality because they tainted and broke the secure environment, so in the long run, and looking at how often blizzard use drop downs, I think it's smart to perhaps make your own drop down system or use some library for such, ugh. :/

Yeah the LUA error when opening the world map is tedious. Turning of the taintlog makes it fade?

To get the error 100% guaranteed:
1) Activate taintlog
2) Go into combat
3) Open the map
4) Enjoy the bug

Doesn't matter if you open the map in or after the combat, this is only the point in time the error seem to start. Because when I open the map out of combat directly after login, I don't get an error. But every single time after the first combat sequence the error comes.

By turning off the taintlog with either /console taintLog 0 or (SetCVar("taintLog", 0) from within an addon) the error is gone.

Of course, without a tainlog, we won't get a taintlog... But currently it's the lesser of two evils as I see it.

EDIT:
I also tried first producing the error, then turning off the taintlog with /console taintLog 0, then trying to reproduce it. And the good news is that a /reload isn't required. The bug stops appearing the instant the taintLog is turned off.